It is used as a gameplay mechanic, as it cannot be destroyed. Steel Spinners function as "stepping stones" for Sonic the Hedgehog’s Homing Attack. They are not harmed by the Homing Attack; instead, they rock from the impact.
The Steel Spinner resembles a Spinner without paint.
If Sonic touches a Steel Spinner as the Green Hover, he is not harmed, but if he touches a Steel Spinner in his regular form, he takes damage.
Steel Spinners do not appear in Sonic Generations; instead, Spinners respawn after being destroyed.